While highly exothermic chemical reactions are widely used to produce a large variety of products, in certain heterogeneous reactions, especially the hydrogenation of coal, the reaction is effected at an elevated pressure and high temperature utilizing a reaction participant (e.g. the coal) in a suspension in an appropriate fluid carrier of particles of a catalyst.
In the usual case, this suspension of catalyst particles in a fluid medium containing at least one reaction participant, is pumped to the reaction pressure and forced into the reactor at which the highly exothermic reaction is carried out. The pressurized suspension is frequently preheated in a plurality of indirect heat exchangers to a temperature close to the desired reaction temperature before being fed to the reaction zone.
At the reaction zone, additional participants can be added if they are not already present in the suspension. In the case of coal hydrogenation and liquefaction, for instance, hydrogen can be fed to the suspension at the reaction zone.
The use of a plurality of indirect heat exchangers for preheating the suspension has two major disadvantages. Firstly, separate indirect heat exchangers are expensive to make, clean and maintain, and have costly components such as heat exchange walls of low corrosivity materials. Furthermore, pressure losses in traversing such indirect heat exchangers are pronounced. Because of these high pressure drops, the pumps and other elements designed to build up the reaction pressure for highly viscous suspensions have high energy consumption.
Thus conventional systems using one or more indirect heat exchangers for preheating viscous suspensions of the type described and particularly suspensions of coal particles in oil containing a catalyst for hydrogenation and liquefaction, are expensive to operate, environmentally detrimental and involve high capital expenditure.